The Manhattan Project Timeline

View of the radioactive plume from the bomb dropped on Nagasaki City
View of the radioactive plume from the bomb dropped on Nagasaki City, as seen from 9.6 km away, in Koyagi-jima, Japan, August 9, 1945. Handout / Getty Images
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Martin Kelly
Martin Kelly
History Expert
  • M.A., History, University of Florida
  • B.A., History, University of Florida
Martin Kelly, M.A., is a history teacher and curriculum developer. He is the author of "The Everything American Presidents Book" and "Colonial Life: Government."
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Updated on June 02, 2020

The Manhattan Project was a secret research project that was created to help America design and build an atomic bomb. The U.S. launched the project in reaction to the startling fact that Nazi scientists had discovered how to split a uranium atom in 1939.

Einstein's Letter

President Franklin Roosevelt was not that concerned when theoretical physicist Albert Einstein first wrote him about the possible consequences of splitting the atom. Einstein had previously discussed his concerns with Enrico Fermi who had escaped from Italy.

However, by 1941 Roosevelt had decided to create a group to research and develop the bomb. The project was given its name due to the fact that at least 10 of the sites used for the research were located in Manhattan. Following is a timeline of the key events related to the development of the atomic bomb and the Manhattan Project. 

Manhattan Project Key Dates
Date Event  
1931 Heavy hydrogen or deuterium is discovered by Harold C. Urey.  
April 14, 1932 The atom is split by John Crockcroft and E.T.S. Walton of Great Britain, thereby proving Einstein's Theory of Relativity.  
1933 Hungarian physicist Leo Szilard realizes the possibility of the nuclear chain reaction.  
1934  Fermi achieves first nuclear fission.  
1938 The Theory of Nuclear Fission is announced by Lise Meitner and Otto Frisch.  
Jan. 26, 1939 At a conference at George Washington University, Niels Bohr announces the discovery of fission.  
Jan. 29,1939 Robert Oppenheimer realizes the military possibilities of nuclear fission.  
Aug. 2, 1939 Einstein writes to President Roosevelt concerning the use of uranium as a new source of energy leading to the formation of the Committee on Uranium.  
Sept. 1, 1939 World War II begins.  
Feb. 23 1941 Plutonium is discovered by Glenn Seaborg , Edwin McMillan, Joseph W. Kennedy, and Arthur Wahl.  
Oct. 9, 1941 FDR gives the go-ahead for the development of an atomic weapon.  
Aug. 13,1942 Manhattan Engineering District is established for the purpose of creating an atomic bomb. This would later be called the "Manhattan Project."  
Sept. 23, 1942 Col. Leslie Groves is placed in charge of the Manhattan Project.  Oppenheimer becomes the project's scientific director.  
Dec. 2, 1942 Fermi produces the first controlled nuclear fission reaction at the University of Chicago.  
May 5, 1943 Japan becomes the primary target for any future atomic bomb according to the Military Policy Committee of the Manhattan Project.  
April 12, 1945 Roosevelt dies. Harry Truman is named the 33rd president of the U.S.  
April 27, 1945 The Target Committee of the Manhattan Project selects four cities as possible targets for the atomic bomb: Kyoto, Hiroshima, Kokura, and Niigata.  
May 8, 1945 War ends in Europe.  
May 25, 1945 Szilard attempts to warn Truman in person concerning the dangers of atomic weapons.  
July 1, 1945 Szilard begins a petition to get Truman to call off using the atomic bomb in Japan.  
July 13, 1945 American intelligence discovers the only obstacle to peace with Japan is "unconditional surrender."  
July 16, 1945 The world's first atomic detonation takes place in the Trinity Test at Alamogordo, New Mexico.  
July 21, 1945 Truman orders atomic bombs to be used.  
July 26, 1945 Potsdam Declaration is issued, calling for the '"unconditional surrender of Japan."  
July 28, 1945 Japan rejects Potsdam Declaration.  
Aug. 6, 1945 Little Boy, a uranium bomb, is detonated over Hiroshima, Japan. It kills between 90,000 and 100,000 people immediately.  
Aug. 7, 1945 U.S. decides to drop warning pamphlets on Japanese cities.  
Aug. 9, 1945 The second atomic bomb to hit Japan, Fat Man, was scheduled to be dropped at Kokura. However, because of poor weather, the target was moved to Nagasaki. Truman addresses the nation.  
Aug. 10, 1945 U.S. drops warning leaflets concerning another atomic bomb on Nagasaki, the day after the bomb was dropped.  
Sept. 2, 1945 Japan announces its formal surrender.  
October 1945 Edward Teller approaches Oppenheimer to aid in the building of a new hydrogen bomb. Oppenheimer refuses.  
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Kelly, Martin. "The Manhattan Project Timeline." ThoughtCo, Apr. 5, 2023, thoughtco.com/the-manhattan-project-timeline-4051979. Kelly, Martin. (2023, April 5). The Manhattan Project Timeline. Retrieved from https://www.thoughtco.com/the-manhattan-project-timeline-4051979 Kelly, Martin. "The Manhattan Project Timeline." ThoughtCo. https://www.thoughtco.com/the-manhattan-project-timeline-4051979 (accessed May 8, 2025).